The intended victim of Wednesday's attack, who was off-duty and not in uniform, was uninjured. The first attacker shouted but the serviceman did not hear what was said because he was wearing headphones.

Detective Superintendent Paul Durham of Norfolk Police said in a statement: "I can confirm we are treating this as an attempted abduction ... The motivation for the attack remains unclear at this time, and whilst we are currently unable to discount terrorism, there are a number of other possible hypotheses."

At a later news conference, Assistant Chief Constable Nick Dean stressed that terrorism was only one theory being examined.

“The motivation of this incident remains unclear. We are unable at this time to discount terrorism but I stress that other lines of inquiry are equally plausible," he explained.

“Vigilance has increased, naturally, as a result of this incident but the wider threat remains unchanged at this time," Dean added.

RAF Marham hasn't been formally used by the U.S. Air Force since the 1960s but is 20 miles from the air base at USAF Lakenheath in Suffolk. It is home to almost 10,000 personnel and their families as well as support workers.

Police seal at Ladywood Road on edge of RAF Marham after serviceman threatened by 2 men with a knife yesterday pic.twitter.com/TRo4LGQcOj